1382 J . Org. Chem., Vol. 65, No. 5, 2000
Britt et al.
with other reactions, estimates of the Arrhenius param-
eters are needed. Unfortunately, there are very few
reports of Arrhenius parameters for the hydrogen ab-
straction of phenoxy radicals with hydrocarbons.11,51-53
Previously, the rate constants for hydrogen abstraction
by phenoxy radicals were assumed to be similar to those
for benzyl radicals.11,53 Thus, for hydrogen-transfer reac-
tions where one center is aliphatic and one center is
benzylic, the rate constant is anchored to the average
value for the reaction between ethyl radical and toluene
log k (per hydrogen, M-1 s-1) ) 7.5 ( 0.2-9.6 ( 0.6/θ.54
If this base reaction is normalized to the hypothetical
thermoneutral reaction,55 we obtain log k (per hydrogen,
M-1 s-1) ) 8.05-15.7/θ. We estimate the 1,5-hydrogen
shift of 1 to 2 to have ∆H°298 ) 8.9 kcal mol-1 and ∆S°298
to -12 cal mol-1 K-1, corresponding to a log A ) 11-12
s-1) than that for the alkyl radical since the aromatic ring
holds the reactive functional groups in close proximity.
Intramolecular hydrogen bonding is well documentated
in salicylaldehyde and guaiacol.59 Theoretical calculations
(MP-SAC2/6-311G**) on the 1,5-isomerization of 2-meth-
yl-1-hexyl radical found ∆Sq ) -7.5 cal mol-1 K-1 60
, which
agreed well with the ∆Sq ) -8.4 cal mol-1 K-1 measured
for the 1,5-isomerization of 3-methyl-2-hexyl radical.61
The preexponential factor for the intramolecular hydro-
gen abstraction by 2-methylbenzoyloxy radical (CH3C6H4-
CO2 ) to form 2-carboxybenzyl radical (•CH2C6H4CO2H)
•
has been measured as 1010.5 s-1 (Tm ) 256 K, ∆Sq ) -12.2
cal mol-1 K-1).62 Although there is a significant variation
between the experimental and theoretical63 preexponen-
tial factors for the 1,5-hydrogen shift, we will use an
average preexponential factor of 1011(1 s-1 (∆Sq ) -10
cal mol-1 K-1). If ∆H°298 and ∆S°298 are split equally
between forward and reverse reaction, and the preexpo-
nential factor of the base reaction is increased to reflect
intramolecular nature of the 1,5-hydrogen shift (1011 s-1),
the rate constant estimated for 1 going to 2 is log k1,2
(s-1) ) 11.3-20.2/θ and the reverse reaction would have
log k2,1 (s-1) ) 10.7-11.3/θ. At 500 °C, log k1,2 ) 5.6 s-1
and log k2,1 ) 7.5 s-1. Thus, the 1,5-hydrogen shift is fast,
and at higher concentrations, it might even compete with
bimolecular reactions (see below).
) 2.6 cal mol-1 K-1 56,57
The preexponential factor for the
.
1,5-hydrogen shift is expected to be similar to that for
other reactions involving 6-centered transition states,
such as the ene or Cope reaction, which typically have
preexponential factors in the range of 1011.5(1.5 s-1 40
. Some
of the earlier experimental investigations into 1,5-
hydrogen shifts of alkyl radicals measured Arrhenius
parameters with preexponential factors of 109.5 s-1 (∆Sq
) -17.5 cal mol-1 K-1), which is lower than expected.58
The loss of entropy in the transition state for the 1,5-
hydrogen shift of 1 is predicted to be smaller (∆Sq ) -7
(51) The rate constant estimates for the reaction of phenoxy radical
with anisole has been reported as log k (M-1 s-1) ) 8.62-16.06/θ,28
but it was assumed that the alkyl radical would be conjugated through
the oxygen to the aromatic ring (i.e., a resonance-stabilized center).
Thus, the rate constant is that reported for the hydrogen abstraction
by the benzyl radical from bibenzyl.31 In the hydrogen abstraction
reaction between tert-butoxy radicals and substituted toluenes and
anisoles, the Hammett plots correlated with σ+ and σ, respectively,
indicating that in the anisoles, direct conjugation with the aromatic
ring is not important. (a) Sakurai, H.; Hosomi, A. J . Am. Chem. Soc.
1967, 89, 458. (b) Sakurai, H.; Hosomi, A.; Kumada, M. J . Org. Chem.
1970, 35, 993.
From the discussion of the reaction pathways of
•
PhOCH2 , it is predicted that 2 will also undergo a 1,2-
phenyl shift to form the o-hydroxybenzyloxy radical (3),
which can lose a hydrogen atom via â-scission to form
salicylaldehyde (log k (s-1) ) 12.5-21/θ), which is the
major pyrolysis product.44 The rate constant for 1 picking
up hydrogen from the reactor walls cannot be deter-
mined, so it is unclear what fraction of 1 forms guaiacol
since it can also be formed by the 1,2-elimination. The
loss of formaldehyde via â-scission of 2 will be competitive
with rearrangement to form 3. Mulder has estimated the
rate constant for â-scission of 2 to be log k (s-1) ) 14-
28.4/θ from the ratio of salicylaldehyde to phenol (3.9:1
at Tm ) 500 °C) produced in the pyrolysis of dimethoxy-
benzene.47 In the FVP of o-CH3O-PPE, the ratio of
salicylaldehyde to phenol is 4.3:1, which is in good
agreement with Mulder’s data. The possibility that
phenol arises from the pyrolysis of salicylaldehyde was
ruled out since independent pyrolysis determined that
it was stable under the FVP conditions. On the basis of
the rate constant for the â-scission of PhCH2O• reported
above, 3 is not expected to contribute to the formation of
phenol. Hence, formation of the major product, salicyl-
aldehyde, and phenol is consistent with the formation of
the key radical intermediate, 2, from 1.
(52) Denisov, E. Handbook of Antioxidants; CRC Press: New York,
1995.
(53) Phenoxy radicals abstract hydrogen atoms slightly faster (1-
6-fold at 375 °C) than benzyl radicals. (a) Stein, S. E. In Chemistry of
Coal Conversion; Schlosberg, R., Ed.; Plenum: New York, 1985; p 13.
(b) Franz, J . A.; Alnajjar, M. S.; Barrows, R. D.; Kaisaki, D. L.;
Camaioni, D. M.; Suleman, N. K. J . Org. Chem. 1986, 51, 1446. (c)
Bockrath, B.; Bittner, E.; McGrew, J . J . Am. Chem. Soc. 1984, 106,
135.
(54) (a) Paputa, M. C.; Price, S. J . W. Can. J . Chem. 1979, 57, 3178.
(b) Daly, M.; Price, S. J . W. Can. J . Chem. 1976, 54, 1814. (c) LaLonde,
A. C.; Price, S. J . W. Can. J . Chem. 1971, 49, 3367. (d) Koski, A. A.;
Price, S. J . W.; Trudell, B. C. Can. J . Chem. 1976, 54, 482. (e) Zhang,
H.-X.; Ahonkhai, S. I.; Back, M. H. Can. J . Chem. 1989, 67, 1541.
(55) By thermochemical balance (∆H°298 ) -12.2 kcal mol-1 and
•
∆S°298 ) 5.2 cal mol-1 K-1) the reverse reaction (PhCH2 + CH3CH3)
has log k (per hydrogen, M-1 s-1) ) 8.6-21.8/θ. Assuming no polar
effects in the hydrogen transfer reaction, the Evans-Polanyi correla-
tion (E ) R(∆H°rxn) + C) can be used with R ) 0.5, i.e., a symmetrical
transition state, and C is determined from the base reaction. To
construct the hypothetical thermoneutral reaction for reaction between
aliphatic and benzylic centers, C ) Ebase - 0.5(∆H°base).
(56) The ∆H°298 of o-CH3OC6H4O• is estimated as -24.7 kcal mol-1
26
from the ∆H°298 of phenoxy radical (11.2 kcal mol-1
)
and group
additivity, -CB-H + CB-O + O-CBC + C-H3C.34, 57 Another method
to estimate this number uses the O-H BDE of guaiacol, (∆BDE 3.9
(59) For recent examples, see: (a) Chung, G.; Kwon, O.; Kwon, T.
J . Phys. Chem. A 1998, 102, 2381. (b) Fujimaki, E.; Fujii, A.; Ebata,
T.; Mikami, N. J . Chem. Phys. 1999, 110, 4238.
(60) Viskolcz, B.; Lendvay, G.; Seres, L. J . Phys. Chem. A 1997, 101,
7119.
(61) Larson, C. W.; Chua, P. T.; Rabinovitch, B. S. J . Phys. Chem.
1972, 76, 2507.
(62) Wang, J .; Tsuchiya, M.; Sakuragi, H.; Tateno, T. Chem. Lett.
1992, 563.
(63) Dorigo, A. E.; McCarrick, M. A.; Loncharich, R. J .; Houk, K. N.
J . Am. Chem. Soc. 1990, 112, 7508. Theoretical calculations (UMP2/
6-31G*) on a series of 1,5-hydrogen atom transfer reactions to oxygen
from the Barton reaction, Norrish Type II reaction, alcohol radical
cation rearrangement, and retro-ene reaction have also found lower
∆Sq (-3.6, -4.8, -7.1, and -4.9 cal mol-1 K-1, respectively) than that
found experimentally.
kcal mol-1, relative to phenol in solution)64 estimated as 83.1 kcal mol-1
in the gas phase,26 and the ∆H°f (guaiacol) ) -58 kcal mol-1
.
The
27
∆H°298 of o-CH3OC6H4O• is estimated as -27.0 kcal mol-1. The ∆H°298
of o-•CH2OC6H4OH is estimated as -17.1 kcal mol-1 from ∆H°298 of
C6H5OCH2 (25 kcal mol-1
)
and group additivity (-CB-H + CB-O +
•
11
O-CBH). Another method to estimate this number uses the BDE of
the OCH2-H bond as 93.3 kcal mol-1, which is assumed to be similar
to that of dimethyl ether.11 The H°298 of o-•CH2OC6H4OH is estimated
as -16.8 kcal mol-1 by this procedure. An average value for the 1,5-
hydogen shift (8.9 kcal mol-1) will be used.
(57) Cohen, N.; Benson, S. W. Chem. Rev. 1993, 93, 2419.
(58) Do´be´, S.; Be´rces, T.; Re´ti, F.; Ma´rta, F. Int. J . Chem. Kinet.
1987, 19, 895 and references therein. The rate constant for the 1,5-
hydrogen shift from 1-hexyl to 2-hexyl radical has been measured as
log k (s-1) ) 9.5-11.6/θ.